FAQ > Functionality Aspect

If you have questions on MEAN WELL’s products, please read the FAQ first. If the listed answers still cannot solve your problems, please contract our local distributors , they should reply to you as soon as received your request.

As a dedicated manufacturer of standard power supplies, MEAN WELL provides a wide variety of power supplies to meet different demands from the markets. However, selecting the right products relies heavily on the correct electrical characteristics and specification, we listed the frequently asked questions for your reference.

Functionality Aspect

Q1

Notes on choosing a switching power supply?

Ans

To increase the reliability of the S.P.S., we suggest users choose a unit that has a rating of 30% more power than actual need. For example, if the system needs a 100W source, we suggest that users choose a S.P.S. with 130W of output power or more. By doing this, you can effectively boost the reliability of the S.P.S. in your system.

We also need to consider about ambient temperature of the S.P.S. and whether there is additional device for dissipating the heat. If the S.P.S. is working in a high temperature environment, we need to make some derating to the output power. The derating curve of "ambient temperature" versus "output power" can be found on our spec sheets.

4. Make sure that the model qualifies for the safety standards and EMC regulations you need.

Q2

Can MEAN WELL's power supply be used in the range of 45Hz ~ 440Hz? If YES, what will happen?

Ans

MEAN WELL's power supply can be used within this frequency range. But if the frequency is too low, the efficiency will also be lower. For example, when a SP-200-24 is operated under 230VAC and rated load, if the frequency of AC input is 60 Hz, the efficiency is around 84%; however, if the frequency of AC input reduces to 50 Hz, the efficiency will be around 83.8%. If the frequency is too high, the power factor of the S.P.S. with PFC (power factor correction) function will reduce and this also will cause higher leakage current. For example, when a SP-200-24 is operated under 230VAC and rated load, if the frequency of AC input is 60 Hz, the power factor is 0.93 and the leakage current is around 0.7mA; however, if the frequency of AC input increase to 440 Hz, the power factor will decrease to 0.75 and the leakage current will rise to around 4.3mA.

Q3

What is minimum load requirement and how can I read it from the spec?

Ans

There are some minimum-load requirements on MEAN WELL's multi-output power supplies. Please read the specification first before connecting to the load. In order to allow the power supply to work properly, a minimum load for each output is required, or else, the output voltage level will be unstable or outer tolerance range. Please refer to “Current range” in the specification as shown in the table below: Channel 1 requires a 2A minimum-load; channel 2 requires 0.5A; Channel 3 requires 0.1A ; Channel 4 does not need any minimum-load.

Q4

Why did the power supply shuts down during operation and after turning it off, I can restart the power supply again?

Ans

In general there are two circumstances that will cause the power supply to shut down. The first one is the activation of the over-load-protection (OLP). To deal with this situation, we suggest increasing the rating of the output power or modifying the OLP point. The second one is the activation of over-temperature protection (OTP) when the internal temperature reaches the pre-set value. All of these conditions will let the S.P.S. enter protection mode and shut down. After these conditions are removed, the S.P.S. will be back to normal.

Q5

What is the control mechanism for cooling fans?

Ans

Cooling fans have a relatively shorter lifetime (typical MTTF, Mean Time To Failure, of around 5000-100000 hours) compared with other components of power supply. As a result, changing operating method of the fans can extend the operation hours. The most common control schemes are shown as below:

Temperature control: if the internal temperature of a power supply, detected by a temperature sensor, is over the threshold, the fan will start working at full speed, whereas, if the internal temperature is less than the set threshold, the fan will stop working or run at half speed. In addition, cooling fans in some power supplies are controlled by a non-linear control method whereby fan speed can be changed with different internal temperatures synchronously.

Load control: if the loading of a power supply is over the threshold, the fan will start working at full speed, whereas, if the loading is less than the set threshold, the fan will stop working or run at half speed.

Q6

What is "Inrush Current"? What will we notice?

Ans

At input side, there will be (1/2 ~1 cycle, ex. 1/120 ~ 1/60 seconds for 60 Hz AC source) large pulse current (20~100A based on the design of S.P.S.) at the moment of power on and then back to normal rating. This "Inrush Current" will appear every time you turn on the power. Although it will not damage the power supply, we suggest not turning the power supply ON/OFF very quickly within a short time. Besides, if there are several power supplies turning on at the same time, the dispatching system of AC source may shut off and go into protection mode because of the huge inrush current. It is suggested that these power supplies start up one by one or use the remote control function of S.P.S. to turn them on/off.

Q7

What is PFC?

Ans

Power Factor Correction or PFC is to improve the ratio of apparent power to real power. The power factor is around 0.4~0.6 in non-PFC models. In models with PFC circuit, the power factor can reach above 0.95. The calculation formulas are as follows: Apparent Power=Input Voltage x Input Current (VA), Real Power= Input Voltage x Input Current x Power Factor (W).
From the point of view of environment friendly, the power plant needs to generate a power which is higher than apparent power in order to steadily provide electricity. The real usage of electricity is defined by real power. Assuming the power factor is 0.5, the power plant needs to produce more than 2WVA to satisfy 1W real power usage. On the contrary, if the power factor is 0.95, the power plant only needs to generate more than 1.06VA to provide 1W real power, It will be more effective in energy saving with PFC function.
Active PFC topologies can be divided into single-stage active PFC and two-stage active PFC, the difference is show as in the table below.

PFC topology

Advantage

Disadvantage

Limitation

Single-stage
active PFC

Low cost
Simple schematic
High efficiency in
small
watt application

Huge Ripple
complex feedback
control

1.Zero “hold up time”. The output is
affected by the AC input directly.
2.Huge ripple current results in lower LED life
cycle.(drive the LED directly)
3.Low dynamic responds, easily affected by
load.

Two-stage active
PFC

High efficiency
Higher PF
Easy feedback control
High adoptive against
load condition

Higher cost
Complex schematic

Suitable for all kinds use

Q8

What is the difference between -V and COM which are marked on the output side?

In MEAN WELL's catalog, we see AC and DC at input, what is it all about?

Ans

Due to different circuit designs, MEAN WELL power supply's input consists of three types as below:
(VAC≒VDC)
a.85~264VAC;120~370VDC
b.176~264VAC;250~370VDC
c.85~132VAC/176~264VAC by Switch; 250~370VDC

In a and b inputs models, power supply can work properly no matter under AC or DC input. Some models need correct connection of input poles, positive pole connects to AC/L; negative pole connects to AC/N. Others may require opposite connection, positive pole to AC/N; negative pole to AC/L. If customers make a wrong connection, the power supply will not be broken. You can just reverse the input poles and power supply will still work.

In c input models, please make sure that you switch the 115/230V input correctly. If the switch is on the 115V side and the real input is 230V, the power supply will be damaged.

Q10

What is MTBF? Is it distinct from Life Cycle?

Ans

MTBF (Mean Time Between Failure) and Life Cycle are both indicators of reliability. MTBF can be calculated by two different methodologies, which are “part count” and “stress analysis”. The regulations, MIL-HDBK-217F Notice 2 and TELCORDIA SR/TR-332(Bellcore) are commonly used to calculate MTBF. MIL-HDBK-217F is a United States military standard, and TELCORDIA SR/TR-332(Bellcore) is a commercial regulation. MEAN WELL utilize MIL-HDBK-217F(Stress Analysis) as the core of MTBF. The exact meaning of MTBF is, after continuously using the power supply for a certain amount of time, the average time that the probability of proper operation is down to 36.8%（e-1=0.368）. Currently MEAN WELL is adopting MIL-HDBK-217F, forecasting the expected reliability through Stress Analysis (excluding fans); this MTBF means the probability of the product can continue the normal work after working continuously up to the calculated MTBF time is 36.8% (e-1=0.368). If the power supply is continuously used at double the MTBF time, the probability of proper operation becomes 13.5%（e-2=0.135）. Life Cycle is found by using the temperature rise of electrolytic capacitors under maximum operating temperature to estimate the approximate life of the power supply. For example, RSP-750-12 MTBF=109.1K hours(25°C); electrolytic capacitor C110 Life Cycle=213K hours (Ta=50℃)

DMTBF(Demonstration Mean Time Between Failure) is a way of evaluate MTBF。Please refer to the following equation for MTBF calculation.

Where

MTBF：Life time determine by specification.

X2：Can be found in chi-square distribution

N：Number of sampling

AF：Acceleration factor, which can be derived from acceleration factor equation.

Ae=0.6

K(Boltzmann Constant)=(eV/k)

T1：Rated temperature of specification. Note: Kelvin will be the unit use for calculation

T2：The temperature that is used in the meaning of acceleration, and the chosen temperature could not result in physical change in materials. Note: Kelvin will be the unit use for calculation.

Q11

What is power good and power fail signals and how can use it?

Ans

Some power supplies provide a "Power Good" signal when they are turned on, and send out a " Power Fail" signal when they are turned off. This is usually used for monitoring and controlling purpose.
Power Good: after the output of a power supply reaches 90% rated voltage, an TTL signal (about 5V) will be sent out within the next 10-500ms.
Power Fail: before the output of a power supply is less than 90% rated voltage, the power-good signal will be turned off at least 1ms in advance.

Q12

I have an TN-1500 inverter. Why the LED indication of AC IN is not lit after mains input has been applied?

Ans

According to the mains voltage of different countries, the output of TN-1500 inverter 110VAC version can be altered to 100/110/115/120VAC. In the same way, TN-1500 inverter 220VAC version can be changed to 200/220/230/240VAC as well. When the inverter is set in UPS mode and the mains voltage fluctuates over ?5% of the set AC output voltage, the inverter will shift its power source from the city power to battery to remain the accuracy of the AC output Voltage. Meanwhile, the AC IN indicator on the front panel of the inverter will be turned off.

Q13

What are the protection forms of overload/overcurrent?

Ans

When current drawn exceeds the rating of the PSU, the protection circuit will be triggered to protect the unit against overload/overcurrent.
Protections of overload/overcurrent can be divided into several forms:
(1)FOLDBACK CURRENT LIMITING
Output current decreases about 20% of rated current, shown as curve (a) in the figure below.
(2)CONSTANT CURRENT LIMITING
Output current remains at a constant level and within the specified range while the output voltage drops to a lower level, shown as curve (b) in the figure below.
(3)OVER POWER LIMITING
Output power remains constant. As output load increases, output voltage decreases in proportion, shown as curve (c) in the figure below.
(4)HICCUP CURRENT LIMITING
Output voltage and current keep pulsing ON and OFF repeatedly when protection is activated. The unit automatically recovers when faulty condition is removed.
(5)SHUT OFF
Output voltage and current are cut off when output load reaches protection range.
NOTE: Protection mode of some of the products combines with different types of the forms mentioned, such as constant current limiting + shut down.
Recover method:
(1)Auto Recovery: PSU recovers automatically after faulty condition is removed.
(2)Re-power on: PSU restarts by manual AC re-power on after faulty condition is removed.
Note：Please do not operate PSU in overcurrent or short-circuit condition for a long period of time to prevent a shorten lifespan or damaging the PSU.

Q14

What is Ripple & Noise? How to measure it?

Ans

It is the small unwanted residual periodic variation of the direct current (DC) output of a power supply which has been derived from an alternating current (AC) source. The wave form is shown as figure below.
There are two AC contents, also known as Ripple and Noise (R&N), on the DC output. The first one, coming from sine wave rectification, is at a low frequency which is 2 times of the input frequency; the second one is at high frequency which is from the switching frequency. For measuring high frequency noise, configurations of an oscilloscope with a bandwidth of 20MHz, a scope probe with shortest ground wire possible, and add 0.1uF and 47uF capacitors in parallel with test point for filtering out noise interference are requires to be made.

Q15

What is Withstand Voltage？How to measure it？

Ans

Indicates Hi-Pot Test or Electric Strength Test. The input should be shorted together as well as the output before test. The test will proceed under particular loop, such as I/P-O/P, I/P-FG and O/P-FG with certain high voltage value for 1 minute. (The typical leakage current is 25mA when testing with AC)

Hi-Pot Test is a way to ensure if the isolation between primary to secondary is done properly, preventing damaging to S.P.S. when facing high voltage between input and output. The test voltage should be gradually increased from 0V to preset level and remains at preset level for 60 seconds with raise time greater than 1 second. In mass production, the test period could be reduced to 1 second. If the leakage current flowing through the isolation material increases rapidly when applying test voltage, it indicates ineffectiveness of isolation (dielectricbreakdown). Corona effect/discharge or transient electrical arc is not considered as failure.

When AC test voltage is applied, Y capacitors are the main cause of leakage current. A 4.7nF capacitor can cause leakage current of 5mA. According to regulations of UL-554, the Y capacitors should be removed for Hi-Pot test, which is not practical for mass production. The only solution is to increase the leakage current setting, typically 25mA, of test instrument. Presently, the criteria of leakage current are not defined in safety regulations.

According to regulations of IEC60950-1, DC test voltage can be substituted when there are bridging capacitors coupled between primary and econdary circuits, so as to solve the problem of leakage current.